Process for coloring aluminum black

ABSTRACT

A process for producing, on aluminium and its alloys, matt black conversion coatings which bond well, are uniform and corrosion resistant. The metal surface is etched with an aqueous iron chloride solution, and then colored in an acidic, aqueous solution of ammonium molybdate and aluminium chloride and finally given an after-treatment with an aqueous, alkaline solution.

United States Patent 1191 1111 3,920,488

Pitton Nov. 18, 1975 PROCESS FOR COLORING ALUMINUM 2,554,256 5/1951 Lewis et al 148/627 BLACK 3,326,728 6/1967 Robinson 148/627 3,391,032 7/1972 Hansen et al. l48/6.27 X [75] Inventor: Oscar Pitton, Schleitheim, 3 799, 5 3/1974 Switzerland 3,833,374 9/1974 [73] Assignee: Swiss Aluminium Ltd., Chippis,

Switzerland Primary Examiner-Ralph S. Kendall Assistant Examiner-John D. Smith [22] Flled' 1974 Attorney, Agent, or Firm--Emest F. Marmorek [21] Appl. No.: 464,328

so Foreign Application Priority Data- [57] ABSIRACT May 17, 1973 Switzerland 7035/73 A process for producing, on aluminium and its alloys, matt black conversion coatings which bond well, are [52] US. Cl 148/627; 148/614 R; 148/6.2; uniform and corrosion resistant. The metal surface is 156/22; 427/287 etched with an aqueous iron chloride solution, and [51] Int. Cl. C23F 7/06 then colored in an acidic, aqueous solution of ammo- [58] Field of Search 148/627, 6.14 R; 156/22; nium molybdate and aluminium chloride and finally 1 17/55; 204/38 A given an after-treatment with an aqueous, alkaline solution. [56] References Cited UNITED STATES PATENTS 11 Clams, N0 Drawmgs 2,285,468 6/1942 Slunder 148/627 PROCESS FOR COLORING ALUMINUM BLACK aluminium and its alloys black. I,

Processes for coloring aluminium black are known, the coloring baths for which processes are based on ammonium molybdates and ammonium chloride. However the coatings obtained by these processes are covered with a dary grey powdery film, and the coating comes off easily when the aluminium is deformed; I

The object of the invention presented here is to produce in a simple way on aluminium and its alloys, black conversion coatings which bond well to the substrate,

are uniform and are corrosion resistant,

The object is fulfilled in terms of the invention in that the metal surface is etched in an iron chloride solution, then coloring in an acidic, aqueous solution of ammonium molybdate and aluminum chloride and finally is The subject of the invention is a processfor coloring 5 centrationof l 20 g/l.

EXAMPLE 1 Sheets of aluminium alloy'containing 4.3 Cu, 1 l Mg, 0.6% Mn, 0.4% Fe and 0.3% Si, and 100 X 70 x 1 mm in dimension, are after degreasing, suspended in a 5% iron chloride solution at 91c for 3 minutes. After given an after-treatment with an alkaline aqueous solution.

be significantly improved, in particular the corrosion resistance against alkalis. v I v Black conversion coatings produced .in accordance with this process, are especially suitable for graphical reproduction processes:

An aluminium sheet which bears a conversion coating or an A1 0 coating after any desired process is covered with a photo lacquer. Those places, which'should later be colored black, are covered with a negative. After exposure to light the lacquer can be removed easily fromthese places, whilst the otherplaceswhich are covered with the polymerised lacquer after develop! ment and the oxide underneath are prevented from being colored black in the process. Tlhusthe black areas, in contrast to thosenot attacked in'the process, can be sharply delineated from areas-which are white or which have another color. This is for example desireable with lettering, scanning or line-structured pictures.

Further features and advantages of the invention appear in the following'description and examples.

The degreased articles which are made of aluminium or an aluminium alloy are first of all etched f6? '1 4 minutes in a :2 '15 aqueous iron chloride solution which is at a temperature of 70F 95C. The articles are then si'uspended for05 10 min in ajcolouri ng-bathat at)" 9 '2 ?c, containing 25 70 g/l, preferably '40 60 g/l ammonium-hepta-molybdate ((NH M6 O .4- H 0) and 10 100 g/l, preferably 4 80 g/l aluminium trichloride. The pH value of the bath-lies between 2 and '5, preferably between 2 and 3 but must never fall below 2.At a bath temperature of 90C the treatment time in the molybdate solution lies preferably between rinsing with cold tapwater the sheets are treated for 3 min in a solution of 5% ammonium hepta molybdate 'and6% aluminium chloride at 91C. The sheets obtain a grey-black coating, With the process in accordance wlth'the' nvention After rinsing in cold tap-water the aluminium sheets are given a l min after-treatment with an aqueous solution of 1.5% sodium chromate and 5% sodium carbonate at a temperature of C. The coatings obtained are smooth and black and exhibit a particularly good bonding base forthe lacquer.

With one liter of coloring bath of the above mentioned composition, sheets of total surface area equal to 1.03 m can be treated without being able to determine a change in colour tone and layer thickness. After 1.05 m however the sheets are no longer black but begin to become increasingly gray and spotty. I

EXAMPLE 2 "This example shows the influence of the after-treatment on the corrosion resistance. Aluminium sheets containing 1.1% Mn, 0.5% Fe, 0.3% Si and 0.1% Cu are part D is anodised and colored black. The corrosion re-' sistance of the treated samples A, B, C and D is tested in the salt spray test (3% NaCl at 35C), the Kestemich test (artificial industrial atmosphere containing 0.66 vol of both S0 and CO and the alkaline test (2% Na PO The results are presented in tables 1 to 3.

Tables 1 and 2 show that sample A, with regard to corrosion resistance in the salt spray and Kesternich test, does not differ basically from sample C (classical alkaline chromatizing process) and D (anodised and then colored black in; accordance with the usual process), although these are among the best known processes. r

From table 3 it follows that anodised sheets (as for example D) have a low resistance to alkalis and, compared with sample A, suffer a large weight loss.

The process in accordance with the invention is therefore with respect to corrosion resistance at least the equal of other processes, but has the advantage over these, that it can be used without problem and without large expenditure for graphical reproduction purposes.

Table l a) Salt-spraytest Time (h) Sample A Sample B Sample C Sample D 72 unchanged attacked unchanged unchanged 2 pits/cm -02 cm 144 hightly attacked strongly attacked unchanged unchanged l-2 pits/cm Change in colour tone,

smaller than 0.2 mm 5 pits/cm from 0.5 to 0.8 mm

216 no change in colour destruction of the slight change in unchanged tone, 2-3 pits/cm coating colour tone from 0.2 to 0.5 mm

480 Change in colour tone, Blank aluminum, slight change in unchanged 4-6 pits/cm no coating left colour tone, from 0.2 to 0.8 mm dark spots Table 2 b) Kesternich Test Time (h) Sample A Sample B Sample C Sample D 27 Colour tone unmore strongly at- Colour tone ununchanged changed, small tached than sample changed, small spots C, change in colour spots tone 54 strongly attached, strongly attached, Change in colour stongly attached, large, white spots marked change in the layer flaked tone, small colour tone, but off spots present very hard, dark grey and rough surface 81 strongly attached, Corrosion layer destruction of the marked change in with large white thicker and harder coating colour tone spots, change in colour tone I08 as after 8! hours Grey corrosion no protective no colour left but rather harder layer even thicker layer left on the sheet. layer and harder oxide layer became soft Table 3 since the kinetics of the coating formation is dependent on the alloy composition. c) Alkali resistance, 2 71 Na PO XA P E 4 Time Anodised sheet, Aluminum Sample? Sample? 45 E M L (h) welgh' blank (mg/1m (mg/1m 'White anodised boards of 99.5 purity aluminium (mg/dm) (mg/dm) are coated with a photosensitive lacquer using a known I 117 36 process, a negative is then laid on the laquer and ex- 5 324 89 33 14 posed to light. After the development only the non- 10 380 98 g2 g8 50 exposed areas are etched until the white oxide skin IS 48 removed, the remaining areas are protected by the polymerised lacquer.

Boards treated this way are given a pre-treatment of EXAMPLE 3 3 minutes in a 5 iron chloride solution at 87C, then This example shows the influence of the time of treatment in the coloring bath on the growth of the layer, at constant temperature. The samples are treated as in example No. l the results are presented in tables 4, 5 and 6. In general almost all aluminium alloys can be colored black. The decisive parameter is the treatment time,

suspended in a colouring bath in accordance with example No. 1 and finally given a very brief after treatment of 0.5 to 1 minute in a 2% aqueous hydrazine solution. Thus the previously non-exposed areas can be colored black, without a change in the anodic layer on the other areas being able to be observed.

Table 4 Properties of the layers obtained in Example 3 Sample No Treatment Layer Weight Colour Table 4- continued Properties of the layers obtained in Example 3 v Remarks Aluminum Sample No Treatment Layer Weight Colour Alloy 1 time (mg/cm) 0.10% Mn 3 2 min 0,678 dark-brown well bonded layer. after treatwas black, in daylight after a few'hours was dark brown f1 3 min i 0.734 brown to black as with sample 3, after several 4 hours of daylight was somewhat brown 5 4 min 0,773 black I no changeof colour tone by y daylight, bonded well 6 6 min 1.176 black as with sample 5, but slightly ,mottled After a treatment time of more than 6 min the layer thickness increases logarithmically. the bond strength becomes worse and the colour tone chan depends on the alloy/used.

ges until it is dark blue. The optimum treatment time at constant temperature Table 5 Properties of the laye'rs'dbiained in' Example i I Aluminum Sample No Treatment Layer Weight Colour Remarks alloy time (mg/cm) i' 3 5 approx "1.5 i' iridescent bonded well 2 10 s L5 "1 1 light brown with, :bonded well. green reflection 4,37: Cu 3 30 5 I5 dark brown bonded well and uniform 1.5% Mg (L671 Mn 0.4% Fe 4 2 min L8 dark brown to conded well and uniform black 5 3 min 2 black bonded well, treatment temperature not above 92C otherwise bluish coating 6 4 min 2 blue-black layer did not bond as well as in the case of No 5 With a treatment time of more than 4 min, the coating becomes dark blue and does not bond well.

Table 6 Properties of coatings obtained in Example No 3 Treatment Layer Weight Colour Remarks Alloy time (mg/cm 1 l 0,285 grey-yellow well bonded layer, very thin 2 30 5 0,25 grey as with sample No l 1.17: Mn 0,571 Fe 0.3% Si 3 2 min 0,399 dark brown to layer not uniform. black after 0,l/ Cu black treatment. after 2*3 days exposure to daylight it became brown 4 3 min 0,698 black with well bonded layer, after 4 5 brown reflec days in daylight became light tion brown 5 4 min 0,763 black bonded well, resistant to colour change in light 6 6 min 1,526 black same as No 7 8 min [.527 black very good. black layer, resistant to colour change in light If the treatment time is longer than 10 min then the layer exhibits white spots and no longer bonds so well. With no after-treatment a white powder forms on the black layer after a few days.

What is claimed is:

l. A process for producing a matte black conversion coating on a metal surface of aluminum or its alloys which coating bonds well, is uniform and is corrosion resistant;

said process comprising the steps of:

etching said surface with an aqueous iron chloride solution; subsequently coloring the etched surface in a coloring bath comprising an aqueous solution containing from about grams to about grams per liter ammoniumhepta-molybdate and from about 10 grams to bath comprising an aqueous alkaline solution at a I 7 grams per liter sodium chromate and from about 30 to about 70 grams per liter sodium carbonate.

4. A process as claimed in claim 1, wherein said aftertreatment bath contains from about 13 to about 17 grams per liter sodium chromate and from about 40 to about 50 grams per liter sodium carbonate.

5. A process as claimed in claim 1, wherein saidaftertreatment bath contains from about to about 20 grams per liter ammonia or an ammonia derivative.

, 6. A process as claimed in claim 5, wherein said ammonia derivative is hydrazine or hydroxyl amine.

7. A process as claimed in claim 1, wherein the temperature of the after-treatment bath lies between 85 and 92C.

8. A process as claimed in claim 1, said etching step further comprising using from about 2 to about percent iron chloride at a temperature of about 70C to about 95C and etching the surface therewith for about LII 10. A process as claimed in claim 1, further comprisingthe step of using said conversion coatings for graphical reproduction processes.

11. A process for producing a matte black conversion coating ona metal surface composed of aluminum or its alloys, which coating bonds well, is uniform and is corrosion resistant;

said process comprising the steps of:

etching said surface with an aqueous solution containing from about 2 to about 15 percent iron chloride for about one to about four minutes at a temperature of about C to about C; subsequently coloring the etched surface from about 0.5 to about 10 minutes in a coloring bath comprising an aqueous solution having a pH value of about 2 to about 5 and containing from about 25 grams to about 70 grams per liter ammonium-hepta-molybdate and from about 10 grams to about grams per liter aluminum trichloride, said coloring bath being at a temperature of approximately 80C to approximately 92C; and thereafter treating the colored surface in an after-treatment bath for about 0.5 to about 3 minutes, said after-treatment bath comprising an aqueous alkaline solution at a temperature of about 80C to about 92C. 

1. A PROCESS FOR PRODUCING A MATTE BLACK CONVERSION COATING ON A METAL SURFACE OR ALUMINUM OR ITS ALLOYS WHICH COATING BONDS WELL, IS UNIFORM AND IS CORROSION RESISTANT; SAID PROCESS COMPRISING THE STEPS OF: ETCHING SAID SURFACE WITH AN AQUEOUS IRON CHLORIDE SOLUTION; SUBSEQUENTLY COLORING THE ETCHED SURFACE IN A SOLORING BATH COMPRISING AN AQUEOUS SOLUTION CONTAINING FROM ABOUT 25 GRAINS TO ABOUT 70 GRAMS PER LITER AMMONIUM-HEPTA-MOLYBDATE AND FROM ABOUT 10 GRAMS TO ABOUT 100 GRAMS PER LITER ALUMINUM TRICHLORIDE; AND THEREAFTER TREATING THE COLORED SURFACE IN AN AFTER-TREATMENT BATH COMPRIISING AN AQUEOUS ALKALINE SOLUTION AT A TEMPERATURE OF ABOUT 80*C TO ABOUT 92*C.
 2. A process as claimed in claim 1, wherein said coloring bath contains from about 40 to about 60 grams per liter ammonium-hepta-molybdate and from about 40 to about 80 grams per liter aluminum trichloride dissolved therein and has a pH value of 2 -
 3. 3. A process as claimed in claim 1, wherein said after-treatment bath contains from about 10 to about 20 grams per liter sodium chromate and from about 30 to about 70 grams per liter sodium carbonate.
 4. A process as claimed in claim 1, wherein said after-treatment bath contains from about 13 to about 17 grams per liter sodium chromate and from about 40 to about 50 grams per liter sodium carbonate.
 5. A process as claimed in claim 1, wherein said after-treatment bath contains from about 10 to about 20 grams per liter ammonia or an ammonia derivative.
 6. A process as claimed in claim 5, wherein said ammonia derivative is hydrazine or hydroxyl amine.
 7. A process as claimed in claim 1, wherein the temperature of the after-treatment bath lies between 85* and 92*C.
 8. A process as claimed in claim 1, said etching step further comprising using from about 2 to about 15 percent iron chloride at a temperature of about 70*C to about 95*C and etching the surface therewith for about one to about four minutes.
 9. A process as claimed in claim 1, wherein said coloring bath has a pH value from about 2 to about 5 and a temperature from about 80*C to about 92*C further comprising the step of coloring said surface from about 0.5 to about 10 minutes.
 10. A process as claimed in claim 1, further comprising the step of using said conversion coatings for graphical reproduction processes.
 11. A process for producing a matte black conversion coating on a metal surface composed of aluminum or its alloys, which coating bonds well, is uniform and is corrosion resistant; said process comprising the steps of: etching said surface with an aqueous solution containing from about 2 to about 15 percent iron chloride for about one to about four minutes at a temperature of about 70*C to about 90*C; subsequently coloring the etched surface from about 0.5 to about 10 minutes in a coloring bath comprising an aqueous solution having a pH value of about 2 to about 5 and containing from about 25 grams to about 70 grams per liter ammonium-hepta-molybdate and from about 10 grams to about 100 grams per liter aluminum trichloride, said coloring bath being at a temperature of approximately 80*C to approximately 92*C; and thereafter treating the colored surface in an after-treatment bath for about 0.5 to about 3 minutes, said after-treatment bath comprising an aqueous alkaline solution at a temperature of about 80*C to about 92*C. 